1. Field of the Invention
The present invention relates to a method and apparatus for solidifying radioactive waste accompanying chlorine recycling or radioactive iodine removal.
2. Description of the Related Art
During the recovery of uranium and transuranic elements using an electrochemical method after nuclear fuel is used, metal halide is used as an electrolyte which contains a fission product and a small amount of actinide. Therefore, the used nuclear fuel is very harmful to the natural environment, and thus should be isolated from the outdoor environment and controlled in a specific repository over a long duration in consideration of its radioactive half-life until its harmful effects disappear.
Since such radioactive waste is highly soluble in water and volatilized at a high temperature of 610° C. or higher, it is difficult to directly apply typical solidification techniques and is also not easy to secure nuclide immobilization. To lower solubility and reduce volatile properties, radioactive metal chloride should be converted to a specific compound which is insoluble and non-volatile. At this time, a material which may react with all types of radioactive metal chlorides should be used as a reactant, and the reaction process should be carried out simply to reduce the generated amount of secondary radioactive waste.
According to a method suggested by the ANL in the USA (Lexa, D., Leibowitz, L., Kropf, J. On the reactive occlusion of the (uranium trichloride+lithium chloride+potassium chloride) eutectic salt in zeolite 4A. J. Nucl. Mater. 2000, 279, 57-64; Lambregts, M., Frank, S. M. Characterization of cesium containing glass-bonded ceramic waste forms. Microporous. Mesoporous. Mater. 2003, 64, 1-9; Ebert, W. L., Lewis, M. A., Johnson, S. G. The precision of product consistency tests conducted with a glass-bonded ceramic waste form. J. Nucl. Mater. 2002, 305, 37-51; Morss, L. R., Lewis, M. A., Lichmann, M. K., Lexa, D. Cerium, uranium, and plutonium behavior in glass-bonded sodalite, a ceramic nuclear waste form. J. Alloy. Compd. 2000, 303-304, 42-48; Lichmann, M. K., Reed, D. T., Kropf, A. J., Aase, S. B., Lewis, M. A. EXAFS/XANES studies of plutonium-loaded sodalite/glass waste forms. J. Nucl. Mater. 2001, 297, 303-312), 10 wt % of molten salt waste and 90 wt % of zeolite are mixed and included at a medium temperature (about 550° C.); 33.3 wt % of borosilicate glass is then added; and thereafter the resulting mixture is thermally treated at about 915° C. to convert radioactive metal chloride to sodalite existing in zeolite. According to the above-described method, the final solidified body has a very large volume and the sodalite structure is collapsed at a high temperature of 915° C. or higher, and thus there is a limitation in terms of temperature.
Also, the SRINR in Russia (Lavrinovich et al., Vitrification of chloride waste in the pyroelectrochemical method of reprocessing irradiated nuclear fuel, Atom. Energy. 2003, 95(5), 781-785; Lavrinovich et al., Combined vitrification of chloride and phosphate waste by pyroelectrochemical reprocessing of nuclear fuel, Atom. Energy. 2006, 101(6), 894-896), suggests a method for immobilizing radioactive chloride into a glass medium at about 950° C. by using phosphate glass is proposed; however, this method still has a disadvantage in that the durability of the solidified body is poor.
The above methods relate to a method of finally disposing all the wastes generated by immobilizing chlorine (Cl) into the solidified body, and are thus problematic in that the disposal volume of the radioactive waste is greatly increased. Also, since Cl, which is a non-radioactive element, is also immobilized, Cl should be continuously supplemented by an amount as much as the amount of discharged metal chloride.
That is, since Cl is disposed as being immobilized with radioactive nuclide, the above methods lead to the problem in which reduced waste loading or the use of a glass medium with low durability is required. Also, the above methods cause other problems in which the total volume of the waste increases and the economy of disposal decreases because Cl should be continuously supplemented.
The present inventors, therefore, solved the problem regarding solidification, and developed a method and apparatus for solidifying radioactive waste accompanying chlorine recycling or radioactive iodine removal, in which the amount of waste is minimized by reducing the total volume of the waste, and finally completed the present invention.